thank you every one for the support by editing and answering I finally did it. since it wasn't easy for me to do this i'm posting the answer to help someone like me :). i didn't change the xml.
php file
$url = '../../data/auction2.xml';
$itemNumber ="4";
$bidDoller = 85;
$bidCent=95;
$xml=simplexml_load_file($url);
$resultDoller= $xml->xpath('//item[itemNumber="'.$itemNumber.'"]/latestBid/bidPrice/doller');
$resultCent= $xml->xpath('//item[itemNumber="'.$itemNumber.'"]/latestBid/bidPrice/cent');
$resultDoller[0][0]=$bidDoller;
$resultCent[0][0]=$bidCent;
print $xml->asXML();
file_put_contents($url, $xml->asXML());